The invention relates to an electrochemical cell comprising a negative electrode, the electrochemically active material of which consists of an intermetallic compound forming a hydride with hydrogen, which compound has the CaCu.sub.5 -structure and the compositional formula AB.sub.m C.sub.n, where m+n is between 4.8 and 5.4, where n is between 0.05 and 0.6, in which A consists of Mischmetall or of at least one element selected from the group consisting of Y, Ti, Hf, Zr, Ca, Th, La and the remaining rare earth metals, in which the total atomic quantities of the elements Y, Ti, Hf and Zr may not be more than 40% of A, in which B consists of two or more elements selected from the group consisting of Ni, Co, Cu, Fe and Mn, the maximum atomic quantity per gram atom of A is for Ni: 3.5, for Co: 3.5, for Cu: 3.5, for Fe: 2.0 and for Mn: 1.0, and in which C consists of at least one element selected from the group consisting of Al, Cr and Si in the indicated atomic quantities: Al: 0.05-0.6, Cr: 0.05-0.5 and Si: 0.05-0.5.
The cell may be in open communication with the atmosphere or may be sealed from the atmosphere. A cell sealed from the atmosphere may have a valve which is proportioned so as to become operative at a previously fixed pressure.
In a rechargeable cell of the closed type the electrochemically active part of the positive electrode may be nickel hydroxide, silver oxide or manganese oxide, nickel hydroxide being generally preferred for practical reasons.
In the cell an electrolyte is used which generally consists of an aqueous solution of one or more alkali metal hydroxides, such as lithium hydroxide, sodium hydroxide and potassium hydroxide, having a pH exceeding 7.
The cell may furthermore comprise a separator which separates the electrodes electrically but permits transport of ions and gas. The separator may consist of synthetic resin fibres (which may or may not be woven), for example of polyamide fibres or polypropylene fibres.
Such an electrochemical cell is described in U.S. Pat. No. 4,487,817. In said Patent the electrochemically active material of the negative electrode is selected so that it is very resistant to corrosion, thereby limiting to the extent possible the deterioration of the electrochemical capacity of the cell.
A disadvantage of the electrochemical cell as described in this U.S. Patent is that the capacity of the cell in the first charge and discharge cycle is less than the maximum capacity and that during the first 20 to 30 charge and discharge cycles it increases only gradually to that maximum capacity. This phenomenon is called "activation".
A further disadvantage of the known electrochemical cell is the relatively low power density of the cell and at a low operational temperature, for example below 0.degree. C. The expression "power density" is to be understood to mean herein the capacity of the cell at high discharge rates. Both properties previously described reduce the usefulness of the stable hydride-forming materials as described in this U.S. Patent.